Incremental position transducers or encoders are of two general types. These are linear and rotary. The rotary type of encoder, as seen in U.S. Pat. No. 3,262,105, does not measure or sense increments of position directly along an axis. It has a rotatable circular scale which is driven by a screw, or the motor which drives the screw, which, for example drives a carriage in an axis. The encoder then senses increments of angular displacement of the scale measured on the periphery of the scale rotor as well as revolutions of the scale. The linear type of encoder, as seen in U.S. Pat. Nos. 2,848,698 and 3,245,144, senses increments of position along the axis of freedom as well as the limits of movement. A linear scale is provided and the encoder resolves scale divisions in the axis upon the occurrence of relative movement between the encoder body and the scale.
Optical or magnetic encoders have been employed. The general principles are the same for both magnetic and optical techniques. Magnetic incremental transducers were employed in a machine tool application in U.S. Pat. No. 3,245,244, above.
The use of dual channels for resolving a single scale as practiced in such prior art arrangements provided a sense of the direction of displacement as well as providing a scale count or scale resolution that was finer than the actual dimensions of the physical scale divisions. Two transducer heads in side-by-side relationship displaced one-quarter of a scale division apart along the scale provided a pair of time varying quadrature phase signals which when processed provide information as to the direction of movement as well as scale resolution equivalent to one-quarter of a physical division of the actual scale.
Such arrangements require equipment and logical processing capabilities which are expensive and complex. Complexity adds to maintenance costs to insure reliability. In less expensive systems such as small letter size printers where such precise scale resolution is not required the expense associated with such system sophistication, both as to initial cost and maintenance may not be justified. System simplification and parts reduction can be expected to improve reliability while reducing initial costs.